Memory metal is an alloy (for example, an alloy of nickel and titanium) of particular near stoichiometric composition which has a memory of a particular stable shape. Memory metal has two structures, depending upon the temperature: the martensitic or cold structure and the austenitic or hot structure. For any given memory metal there is a temperature above which the metal has an austenitic structure and another, lower, temperature below which the metal has a martensitic structure. Between these two structures, there is a temperature area or range known as the transformation temperature range, in which the alloy is transformed. When heated, the alloy transforms from martensite (the "cold structure") to austenite (the "warm" structure). When cooled, the alloy transforms from austenite to martensite. These transformations take place with a certain hysteresis or lagging effect.
FIG. 1 is a stress strain curve for a memory metal. As shown in FIG. 1, when the memory metal is at a temperature below the transformation temperature range (TTR), the memory element has a martensitic structure and is easily deformed. Specifically, as shown in the stress-strain curve of FIG. 1, when a tensile force (F) is applied to the memory element at a temperature below the TTR, the strain increases linearly in area AB according to Hooks law, i.e., stress and strain are directly proportional. However, strain remains constant in the area BC as the metal deforms up to a maximum value of 8 percent. When the deformation force is removed, there remains an apparent plastic deformation, represented by AD. As shown in FIG. 1, the lengthening occurs in response to a relatively small force F.sub.3 since the martensitic structure is easily deformed.
When the temperature is above the transformation temperature range (TTR), the memory element has an austenitic structure and it has stable dimensions (a conditioned shape). When a memory element deformed at a temperature beneath TTR is heated, it will return (i.e., shrink) to its conditioned shape or dimensions. The return to the stable shape takes place with a force that is considerably higher than the force needed to deform the memory element at a temperature beneath the TTR. This is apparent from FIG. 1 which shows that the tensile curve representing the recovery force F.sub.2 (the "hot tensile curve") lies much higher than the curve representing the deformation force F.sub.3 (the "cold tensile curve"). Therefore, when the memory element is heated, an effective force of F.sub.2 minus F.sub.3 remains. This is the net force acting to return the memory metal to its stable shape. In the case of a memory metal element having a measurable length, the difference between the deformed length of the memory metal when it is cold and length of the memory metal when it is hot is referred to as the stroke. When the stroke of the memory element (spring) ranges from C to B, the amount of work, done by the memory element, is represented by the surface area described between the hot and cold tensile curves. The amount of work will be (F.sub.2 -F.sub.3).times.(.epsilon..sub.C -.epsilon..sub.B) and this can be used to cause a movement with a certain force. Thus, memory metal is an energy converter. It transforms heat directly into mechanical energy.
Previous attempts have been made to use temperature sensitive materials in actuators. An example is the temperature responsive ventilator disclosed in U.S. Pat. No. 3,436,016 to louvers or shutters associated with the frame for closing the framed area in one position and opening the framed area in another position. A temperature-responsive spring is connected to the louvers or shutters. In response to temperature changes, the spring positions the shutters or louvers between the opened and closed positions.
U.S. Pat. No. 4,497,241 to Ohkata discloses a device for automatically adjusting the angle of a louver. The device includes a memory metal spring for applying a rotary force to the louver in one direction and a bias spring for applying a rotary force louver in the opposite direction. The position of the louvers is determined by the balance between the memory metal spring and the bias spring. When the air is cold, the memory metal spring is deformed by the bias spring. Conversely, when the air is warm the memory metal spring returns to its memorized position against the bias spring, and the louver rotates to a position aligned with the passage. In this way, the louver is automatically controlled in response to the temperature of the diffused air.
All of the devices disclosed in the various embodiments of the Ohkata patent include a counterbalancing spring 6, which does not have a constant spring force; consequently, the spring provides an increasingly strong resistance force as it is biased. As disclosed in greater detail below, the present inventors have discovered that the use of a spring which does not have a characteristic with a constant force can severely limit the stroke of the actuator and thus limit the usefulness of the actuator itself.